Our '05 is at 170K with no engine-specific problems thus far. We've had it since new and it seems to run strong and solid. We have followed the factory (not dealer) maintenance schedules over its life and replaced the shocks and struts at 150K (not that they were really bad, but it just seemed like the right thing to do after so many miles).
I'd prefer better MPG, we see 20-21 on the highway running 75MPH, but for this size of vehicle and V6, that's probably not so bad. (New gen MDXs are only rated at 21 on the highway).
Since we like the powertrain and overall vehicle so much and we probably wouldn't get much if we sold it, we recently decided to keep the MDX until it starts encountering super serious problems - perhaps anything over $4K or so. I test drove a '10 Venza last week and the road noise from the rear was terrible - even compared to our '05 gen MDX.
My current outlook is that we should see well over 200K with this vehicle. I am prepared to do misc DIY fixes for minor issues as they crop up. Failed switches, bulbs needing replacement, power seat gremlins, DVD player failures (~$250 for rebuilt unit), etc. are all issues I am willing to fix on my own as we keep our vehicle past 200K.
The main problem in terms of the overall drivetrain has been the transmission - a common and well documented problem with MDXs. At 130K or so the dealer replaced it with a rebuilt unit and split the cost with us we have been good customers at this dealer. Our cost was $1,500 or so.
The resonating sound at low speeds and low RPM is a common early symptom of this problem. In our case is got progressively worse over 20K miles or so until it was clear something serious was afoot. Even with the rebuilt tranny we hear slight resonating under these conditions. Maybe we can expect another failure in 50-100K. (I've read that a lot of the failures have occurred well under 100K).
As you look at MDXs, it might be useful to inquire whether the transmission has been replaced or rebuilt and pay close attention to the drivetrain sounds at 1,200 or so RPM and 30-45 MPH as you apply some light load.